The present invention relates to a transmission system comprising a transmitter having an audio encoder, said audio encoder comprising segmenting means for deriving at least first signal segments and second signal segments from an input signal representing an audio signal, the first signal segments being longer than the second signal segments, the audio encoder comprising means for deriving an encoded audio signal from said first and second signal segments, the transmitter comprising transmit means for transmitting the encoded audio signal to a receiver via a transmission medium, the receiver comprising receive means for receiving the encoded audio signal from the transmission medium, the receiver further comprising an audio decoder for deriving a decoded audio signal from the encoded audio signal.
The present invention is also related to a transmitter, an encoder, an encoding method, a tangible medium carrying a computer program for performing an encoding method, and a signal carrying a computer program for performing an encoding method.
A transmission system according to the preamble of claim 1 is known from U.S. Pat. No. 5,886,276.
Such transmission systems and audio encoders are used in applications in which audio signals have to be transmitted over a transmission medium with a limited transmission capacity or have to be stored on storage media with a limited storage capacity. Examples of such applications are the transmission of audio signals over the Internet, the transmission of audio signals from a mobile phone to a base station and vice versa and storage of audio signals on a CD-ROM, in a solid state memory or on a hard disk drive.
Different operating principles of audio encoders have been tried to achieve a good audio quality at a modest bit rate. In one of these operating methods, an audio signal to be transmitted is divided into a plurality of segments, normally having a fixed length of 10-20 ms. In each of said segments the audio signal is represented by a plurality of signal components, which can be sinusoids that are defined by their amplitudes, their frequencies and possibly their phases.
The transmitter transmits a representation of the amplitudes and frequencies of the signal components to the receiver. The operations performed by the transmitter can include channel coding, interleaving and modulation.
The receive means receive a signal representing the audio signal from a transmission channel and performs operations like demodulation, de-interleaving and channel decoding. The decoder obtains the representation of the audio signal from the receive means and derives a reconstructed audio signal from it by generating a plurality of sinusoids as described by the encoded signal and combining them into an output signal.
A problem with these audio encoders is to select a proper length (in units of time) for the signal segments. If the signal segments are long, a good frequency resolution for the determination of the signal components is possible, but, as a result of a limited time resolution, a phenomenon called pre-echo can occur. Pre-echoes occur when an event such as a sudden attack of an audio signal is already audible prior to the actual occurrence of the event. If the signal segments are short no problems with pre echoes occur, but the frequency resolution for the determination of signal components with low frequencies is drastically reduced.
To improve this, in the above US patent, the input signal is split into a number of sub-bands by means of a sub-band filter and for each of the sub-bands a different length of the signal segments is chosen. The length of the signal segments is chosen inversely proportional to the frequency range of the corresponding sub-band.
A problem with this approach is that the encoding quality for signal components located around the transition band of the sub-band filter is less than for other signal components.
An object of the present invention is to provide a transmission system according to the preamble in which the above problem is solved.
To achieve the above object, the transmission system according to the invention is characterized in that the encoding means comprise preventing means for preventing multiple representations of a single signal component to occur in the encoded audio signal.
The present invention is based on the recognition that in the prior art system frequencies in the transition bands of the sub-band filter lead to multiple representations of the same signal components of the input signal. These multiple representations are undesired when a psycho-acoustical model is used to determine the signal components to be transmitted. Furthermore it is difficult to reassemble a signal component which is represented twice in the encoded signal. The multiple representations also lead to a larger bitrate than would be present without multiple representation of a signal component.
By using preventing means to prevent or suppress these multiple representations of a single signal component, the associated problems are also eliminated.
In an embodiment of the present invention, the preventing means comprise synthesis means for deriving a synthetic audio signal from a part of the encoded audio signal representing said first signal segments and subtraction means for deriving the second signal segments by subtracting the synthetic audio signal from a signal representing the input signal. By subtracting a synthetic audio signal representing the first signal segments from a signal representing the audio signal to obtain the second signal segments, it is realized that signal components determined from the first signal segments are removed from said signal representing the audio signal. Consequently, these signal components are not or strongly attenuated present in the second signal segments. In this way a multiple representation of said single signal components is avoided.
In a further embodiment of the invention, the segmentation means are arranged for deriving further signal segments from the input signal, said further signal segments being longer than the first signal segments, the audio encoder being arranged for deriving the encoded audio signal also on basis of the further signal segments, the audio encoder further comprises synthesizing means for deriving a further synthetic signal from a part of the encoded audio signal representing said further signal segments and subtraction means for deriving the first signal segments by subtracting the further synthetic audio signal from a signal representing the input signal. Experiments have shown that it advantageous to use successive segments with at least three different lengths, because the number of periods in a segment may not be too large but also not too small.
In a still further embodiment of the invention, the audio encoder comprises a filter for deriving a filtered signal from the input signal and in that the audio encoder is arranged for deriving the first signal segments from the filtered signal. By filtering the input signal it is possible to remove some signal components from said input signal making the determination of the remaining signal components more reliable. The signal components not present anymore in the first signal segments are present in the second (or further) signal segments where they are determined. Consequently still a complete description of the output signal is obtained.
A still further embodiment of the invention is characterized in that the coding means are arranged for representing amplitudes on a psycho-acoustical relevant scale. The use of a psycho-acoustical relevant scale to represent amplitude results into a more efficient use of the transmission channel, because less symbols are needed to represent a signal with a given dynamic range. Such a psycho-acoustical relevant scale can e.g. be a logarithmic scale.